Water - H₂O - is the universal solvent. Pool water chemistry involves keeping the water in balance and proper sanitation to prevent the growth of bacteria and algae. Balance means that the water is saturated with minerals and is neither corrosive or scale forming. If the water lacks the minerals to be in balance, it will try to satisfy the demand by removing the calcium from the pool surface and other metals from the pool equipment. If the water becomes oversaturated, the calcium will precipitate out in the form of calcium carbonate on the pool surfaces.

There are several indexes that are used to determine water balance, however, the Langlier Index is the most commonly used. Dr. Wilfred Langlier, who was trying to find ways to prevent the build up of calcium carbonate in industrial water treatment in the 1930's, discovered five factors that controlled water balance. They are Calcium Hardness, Total Alkalinity, pH, temperature, and Total Dissolved Solids. Below is a simplified version for use in swimming pools. An average factor for Total Dissolved Solids (TDS) is included. TDS should not exceed 2000 ppm.

Saturation Index = pH + TF + CF + AF - 12.1

TF (temperature factor), CF (calcium factor), and AF (alkalinity factor) are values may be obtained from the following charts.

For example, let's assume a pH of 7.5, a temperature of 84°F, calcium hardness of 300 ppm, and alkalinity of 100. The corresponding factors from the chart are .7 TF, 2.1 CF, and 2.0 AF. Using the equation, 7.5 + .7 + 2.1 + 2.0 - 12.1 = +0.2 Values for the Saturation Index are considered satisfactory in a swimming pool if the value is between plus 0.3 and minus 0.3.

The stablizer (cyanuric acid) used to stablize chlorine in pools to prevent the sun and heat from rapidly dissapating chlorine effects the reading obtained for the alkalinity by falsely increasing the alkalinity reading. The effect varies depending upon the pH of the water. Between pH readings of 7.0 and 8.0, your total alkalinity must be reduced by a quantity equal to 21% - 36% of the amount your cyanuric acid concentration. The relationship between the pH and cyanuric acid (CYA) is expressed in this table.

Example: For a pH of 7.6 and a CYA of 80 ppm. multiply .32 X 80 = 25.6. Subtract 25.6 from the alkalinity of 100 and the true alkalinity is 74.4. This reduces the AF factor by 0.1.

Recommended Ranges

How to correct for an imbalance.

The first step is to adjust the Total Alkalinity. Total Alkalinity measures the ablility of the water to resist changes to the pH when acids or alkaline chemicals are added to the water. If the alkalinity is low, the pH may also stay on the low side and the addition of acid may cause it to drop into the acidic range. If the alkalinity is high, the pH may also be high and the water could become cloudy or scale forming. The addition of acid lowers alkalinity and bicarbonate of soda (baking soda) raises alklinity.

To raise Alkalinity using Baking Soda (Sodium Bicarbonate, 100%)

To Decrease Alkalinity Using Dry Acid (Sodium Bisulfate, 93.2%)

To Decrease Alkalinity using Muriatic Acid (20⁰ Baume / 31.45%)

Muriatic acid is used to lower both the pH and Total Alkalinity (TA). The amount of acid from the tables should be added over time and not all at once. Initially add 50%, 25%, 15%, and the final 10% at 24 hours intervals, and check the pH between each addition.

The next step is to adjust the Calcium Hardness. If the calcium level is to low, the water becomes aggressive and tries to dissolve the calcium from whatever source it can. It could etch the pool plaster, pit the concrete, dissolve the tile grout, or take if from other metal sources in contact with the pool water. If the calcium level is too high, the water could be cloudy, and cause formation of scale on the surfaces of the pool.

The calcium level is raised by adding Calcium Chloride. Lowering is accomplished by dilution, provided the calcium level of the source water is lower. If it is not, the next best alternative is to use a sequestering agent. Calcium is a metal and the sequestering agent will bond with the calcium ion and keep it solution.

To Increase Calcium Hardness using Calcium Chloride (77%)

Retest the pH, Total Alkalinity, and the Calcium Hardness and make any final adjustments. The water should now be in balance.